1. Field of the Invention
The present invention relates to a sheet conveying apparatus having a U-turn path.
2. Related Background Art
FIG. 6 is a perspective view of a conventional sheet conveying apparatus having a so-called U-turn path for greatly changing a conveying direction of a sheet. This apparatus comprises a convey roller 100 rotated in a direction shown by the arrow R100, a convey guide 101 having an outer guide surface 101a facing to an outer peripheral surface 100a of the convey roller 100, and a pinch roller (convey sub-roller) 102 disposed downstream of the convey guide 101 in a rotating direction of the convey roller 100. Between the outer peripheral surface 100a of the convey roller 100 and the outer guide surface 101a of the guide 101, there is disposed an arc-shaped U-turn path G through which the sheet is passed. The pinch roller 102 is rotatably mounted on a front end of a pressure plate 103 rocked around a shaft 103a, and a rear end of the pressure plate 103 is biased upwardly by a pair of tension springs 104 so that the pinch roller is urged against the outer peripheral surface 100a of the convey roller 100 with moderate pressure from the above. Incidentally, a downstream outlet Gb of the U-turn path G is defined as a straight gap formed between the generatrix of the convey roller 100 and a terminal edge 101b of the outer guide surface 101a of the guide 101.
The sheet fed to an inlet Ga of the U-turn guide G from a direction shown by the arrow A is conveyed through the U-turn path G by the convey roller 100 rotating in the direction R100. When a tip end of the sheet leaves the outlet Gb, the tip end of the sheet reaches the pinch roller 102 while being guided by a lower surface of the pressure plate 103. When the tip end of the sheet reaches the pinch roller 102, the sheet is conveyed by the convey roller 100 and the pinch roller 102. As the sheet is further conveyed, a trail end of the sheet passes through the outlet Gb of the U-turn path G and then leaves through the pinch roller 102. Then, the sheet is conveyed to, for example, a downstream image forming portion.
However, in the above-mentioned conventional sheet conveying apparatus, when a sheet (for example, a thick sheet) having great resiliency is conveyed and image formation is effected regarding the thick sheet at the downstream image forming portion, there arose a problem that a good image could not often be formed. That is to say, as shown in FIG. 7, since the sheet P having great resiliency is hard to be bent or curved, when the sheet is being conveyed through the U-turn path G, it is forcibly curved in accordance with a curvature of the U-turn path G. As a result, the tip and trail ends of the sheet rub the outer guide surface 101a of the guide 101. And, when the trail end Pb of the sheet P leaves the outlet Gb of the U-turn path G, i.e., when the trail end Pb is disengaged from the terminal edge 101b of the outer guide surface 101a of the guide 101, since the trail end Pb of the sheet P is abruptly returned from a posture shown by the solid line to a posture shown by the broken line by the resiliency (storing force) of the sheet P, a convey amount of the sheet P becomes unstable due to the shock, thereby worsening the image quality.
For example, as shown in FIG. 7, when a recording apparatus 105 includes a recording means 106 disposed immediately downstream of the pinch roller 102, since the sheet P is intermittently conveyed by the convey roller 100 by a predetermined amount and one-line recording is effected by the recording means 106, if the convey amount of the sheet P does not coincide with the recording width (in the conveying direction), a recording area will be deviated from a correct one, so that the desired image cannot be obtained. If the convey roller 100 is stopped in a condition (as shown by the solid line) that the trail end Pb of the sheet P is slightly caught by the terminal edge 101b of the outer guide surface 101a, the trail end Pb of the sheet is snappingly disengaged from the terminal edge 101b to reach the posture shown by the broken line. In this case, due to the presence of any play in a drive transmitting system for the convey roller 100 and any plays in bearings, the desired convey amount cannot be obtained. To eliminate such inconvenience, there is proposed a technique in which a braking means is incorporated into the drive transmitting system to minimize influence of the play. However, in this case, since load torque is increased and greater motor torque is required, a desired result cannot be achieved.
As another proposal, there has been provided a technique in which a flexible sheet member (for example, PET sheet member) is adhered to the guide 101 so that a buffer (provided by the flexible sheet member) is formed between the terminal edge 101b of the outer guide surface 101a and the pressure plate 103, to thereby eliminate the above drawback. However, in this case, if the sheet member is not adhered to the guide at a correct or accurate position, the sheet jam may frequently be caused. Thus, the accuracy of adhesion must be maintained, which in turn worsen the operability.